It's not really just luck, Rooney was always quite productive player so it's not a coincidence he was involved in some goals, but Mata's assist again Middlesbrough was probably more important than any of Rooney's and Martial's two combined this season. I mean, Rooney has at least 3 assists this season where he passes the ball to a player who either scores long range goal or who beats few player and scores a goal(there was somehwere video of his all assists this season). Martial also has 2 assists like that in first game of the season.
It's not really luck, but you cannot call it being creative either, that's why assist don't really always tell you who the most creative player is. Mata should have one or two more assists this season though, altough he hardly picks any assists when the games are over, so that's on him.